A known hybrid electric vehicle powertrain with split power flow paths is disclosed in U.S. Pat. No. 7,285,869, which is assigned to the assignee of the present invention. That powertrain includes an electrical power source including a traction motor, a battery, and an electric generator that is capable of functioning as a motor. An engine is a mechanical power source, and a planetary gear system establishes divided torque delivery paths between the electrical and mechanical sources of power and vehicle traction wheels. The planetary gear unit includes a ring gear connected drivably to the traction wheels, a sun gear drivably connected to the generator, and a carrier drivably connected to the engine. The generator, the motor, and the battery are electrically coupled.
Engine torque applied to the carrier during forward drive in a divided power delivery mode is in a direction that is opposite to the direction of reaction torque applied to the sun gear by the generator. The torque applied to the carrier by the engine is equal to the sum of the torque applied by the engine to the ring gear and the torque applied to the sun gear.
If the electric motor is used during an electric motor launch with the engine off, the motor will drive the ring gear in the same direction as the direction of rotation of the motor rotor. Since the carrier is directly connected to the engine, which is not fueled at that time, the sun gear rotates relatively freely while the carrier is not turning. The planetary gear unit at that time is essentially without a reaction element, except for a small torque delivered to the sun gear by reason of bearing friction losses and gear friction losses.
If the generator is commanded to provide assistance to the motor during a forward motor launch, the generator is controlled to function as a motor as it rotates in a negative direction (e.g. counterclockwise as viewed from the engine's location). At that time, torque is applied to the carrier in a negative direction. An overrunning coupling provides reaction torque at that time so that the generator torque can be transmitted to the ring gear, which drives the traction wheels in a forward driving direction. The battery then provides power to both the motor and the generator. However, the generator may not be able to assist the motor during a reverse launch since there is no reaction torque available to allow generator torque to be distributed to the ring gear.
Since positive engine torque (e.g. clockwise torque as viewed from the engine's location) will reduce the drive torque contribution from the motor, a known strategy utilized by a vehicle system controller may minimize the use of the engine during a reverse driving operation. Under such circumstances, there is a limited operating range in which the engine may be used to supply driving torque to the generator to charge the battery when the battery does not have a sufficient state-of-charge to supply power to the motor.
A peak reverse wheel torque is an important factor that determines a vehicle's ability to climb a steep grade, or to maneuver the vehicle in reverse in certain off-road conditions, or to maneuver the vehicle in reverse over obstacles, such as a roadway curb. However, a powertrain of the type disclosed in U.S. Pat. No. 7,285,869 can develop only a limited peak reverse drive wheel torque.
U.S. Pat. No. 7,576,501 teaches a strategy that uses hybrid powertrain components in such a way as to permit the engine to be motored in an unfueled state to boost reverse driving torque. Briefly, the strategy includes motoring the engine unfueled via a generator during reverse drive operation. With the engine unfueled, negative engine brake torque may be equal to the engine's pumping and friction loss multiplied by the gear ratio. In reverse drive, this may boost peak reverse wheel torque to a value beyond the ability of the traction motor to create reverse wheel torque. U.S. Pat. No. 7,576,501 further teaches selecting a speed at which the engine is motored unfueled via the generator. Because the battery supplies electrical power to provide enhanced, or boosted, reverse wheel torque, power draw is minimized to prolong boosted reverse drive operation. Thus, U.S. Pat. No. 7,576,501 teaches minimizing power consumption by running the engine at its lowest possible engine speed.
The inventors herein have recognized potential issues with such a solution, and have developed systems and methods to at least partially address them. In one example, a method is provided, comprising accelerating an engine coupled to wheels of a hybrid motor vehicle, the engine accelerated via a generator temporarily functioning as a motor, responsive to a driver-requested negative wheel torque that exceeds a capability of an electric motor, also coupled to one or more of the wheels, to provide the negative wheel torque; and operating the engine based only on driver-demand responsive to vehicle speed increasing beyond a threshold vehicle speed. In this way, a vehicle speed may be increased in reverse to a threshold speed that may enable the vehicle to overcome an obstacle or a grade that is preventing the vehicle from achieving the threshold speed.
As an example, accelerating the engine via the generator may provide engine braking torque. As another example, the engine may be accelerated at a maximum rate to provide the negative wheel torque.
In another example, in response to accelerating the engine, and further responsive to an indication that the vehicle speed does not increase beyond the threshold vehicle speed, the method may include decreasing engine speed until the engine speed is below a threshold engine speed, and may further include repeating the accelerating the engine to increase vehicle speed to or beyond the threshold vehicle speed. For example, decreasing engine speed and repeating the accelerating the engine may be conducted any number of times while the driver-requested negative wheel torque exceeds the capability of the motor. In some examples, accelerating the engine may be conducted with wheel brakes released, the wheel brakes configured to provide braking torque to the wheels, and where the wheel brakes are applied in response to the vehicle speed not increasing beyond the threshold vehicle speed, and just prior to decreasing engine speed.
As another example, the vehicle may comprise a powersplit hybrid, the powersplit hybrid including the engine, the electric motor, the generator, a battery, and gearing forming separate power flow paths during forward drive to the wheels from the engine and the electric motor, and from the electric motor and the generator during reverse drive. The electric motor may operate in one direction during forward drive and in a direction opposite to the one direction during reverse drive. Still further, accelerating the engine via the generator may be conducted in the absence of the engine combusting air and fuel.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.